Chemokines and their G-protein coupled receptors constitute a unique ligand-receptor system that governs leukocyte migration and effector function. Chemokines are present in most multicellular organisms, to mediate organ patterning during development. In mammals, chemokines also regulate the function of a sophisticated, flexible host-defense system. The central nervous system (CNS) lesions of multiple sclerosis (MS), an inflammatory demyelinating disease, contain specific populations of activated leukocytes, including monocytes, macrophages, microglia and lymphocytes. Understanding the molecular determinants of leukocyte trafficking to the CNS will promote progress towards effective treatment of MS. Data obtained in the previous round of funding supported our core hypothesis that individual chemokine/receptor pairs are selectively involved in the recruitment and activation of specific leukocyte populations in the CNS. Using experimental autoimmune encephalomyelitis (EAE) as a model system, we propose to examine our hypothesis in detail. We have carefully selected genetically-modified mice that will allow us to examine chemokine signaling in the critical cell populations implicated in EAE and MS. Using these powerful reagents, we intend to clarify how chemokines and their receptors govern the CNS recruitment and activation of diverse cell populations that take part in EAE: monocytes, lymphocytes, astrocytes, NK cells and microglia. The Specific Aims address the following questions: 1. How do MCP- l/CCL2 and CCR2 govern cell recruitment and activation in the CNS? 2. How do fractalkine/CX3CL1 and CX3CR1 regulate the recruitment and activation of resident microglia and infiltrating cells in the CNS of mice with EAE? 3. How do CXCR3 and its ligands govern the accumulation and distribution of T-cells in the CNS of mice with EAE?